New approaches to antimicrobial drug discovery are urgently needed to combat untreatable infections caused by antibiotic resistant or persistent, antibiotic tolerant, bacteria. The long-term goal of this proposal is to develop novel therapy options that may prevent or reduce the complications of human bacterial acute and chronic or relapsing infections, and that could serve as alternatives or adjuncts to antibiotics. To achieve a paradigm shift in antimicrobial therapy, we propose to develop inhibitors of bacterial signaling pathways that control bacterial virulence and antibiotic tolerance mechanisms. Population density-dependent signaling, generally referred to as quorum sensing (QS), is one such mechanism. QS regulates multiple aspects of virulence. It is important for the development of acute infections and as recently discovered for the formation of antibiotic-tolerant cell populations, a process underlying pathogen persistence in chronic infections. This renewal application will test the hypothesis that novel inhibitors of QS, previously identified by us, can lead to the development of highly potent compounds with anti-infective activity in vivo. We will test this hypothesis by employing Pseudomonas aeruginosa, a recalcitrant Gram-negative bacterium that defies eradication by antibiotics and exemplifies current problematic pathogens in hospitals and intensive care units. In the last funding cycle, we demonstrated that P. aeruginosa pathogenesis can be disrupted in vivo by pharmacologically interfering with the multiple virulence factor regulator (MvfR) regulon, a component of QS circuitry that controls virulence and, as we recently discovered, the formation of antibiotic tolerant cells. We elucidated the mechanism of MvfR regulon activation and identified several small chemical compounds that inhibit the MvfR transcription factor and/or interfere with MvfR regulon activity in vivo. Using these chemical compounds, we will test our hypothesis through three Specific Aims: 1) To study the mechanisms of action of the candidate compounds using biochemical, mass spectrometric, and molecular genetic analyses. 2) To improve the most potent QS inhibitors through structure-activity relationship (SAR) studies. 3) To validate the in vivo efficacy of SAR-improved inhibitors in attenuating P. aeruginosa infection in suitable animal models. QS signaling circuits are evolutionarily conserved and play central roles in modulating virulence mechanisms in many different human pathogens. Therefore, by selectively interfering with QS, our data should yield paradigmatic insights that will be generally relevant for the development of new classes of anti-infectives that could limit development of multi-drug resistance and antibiotic tolerance in bacterial pathogens, while preserving beneficial commensal bacteria. Moreover, the study and inhibition of mechanisms involved in antibiotic tolerance could have a major impact on elucidating this unresolved phenomenon, as well as enabling the discovery of the first probe compounds targeted against antibiotic tolerant cells.